It is becoming increasingly common for irrigation installations to be operated intermittently, i.e. the irrigation water is supplied to the installation at regular intervals and for accurately predetermined periods of time. In this way it is ensured that the agricultural crops being irrigated receive an accurately predetermined amount of irrigation water and additives. However, in order to achieve this end it is necessary to ensure that the actual periods during which irrigation water is supplied to the emitter units are strictly predetermined and controlled.
For this purpose the irrigation supply is fed to the installation in predetermined pulses of predetermined time lengths. However, it will be readily realised that each irrigation pulse cannot instantaneously rise to its maximum value upon the initiation of the pulse and similarly it cannot instantaneously die down to zero value upon termination of the pulse. In consequence, the irrigation pulse fed to the unit has got a finite build-up time, a finite peak time and a finite decay time, whilst the irrigation feed to the crops is calculated solely on the basis of the peak time. The fact that irrigation water and the additives continue to be fed (leak) to the crops during the build-up and the decay time may lead to undesirable results. Thus, for example, during the build-up and decay times the plants in the immediate vicinity of a rotary sprinkler will continue to receive irrigation water and additives whilst other plants, which are normally within the range of the sprinkler, will not.